Household, urban or hospital waste usually contains large amounts of organic residues, in particular organochlorine residues, and heavy metals (in particular zinc, lead, cadmium, mercury and chromium). It also often contains metal chlorides (particularly sodium chloride) and aluminium. A technique regularly used to reduce the volume of this waste consists in incinerating it. The incineration of such waste leads to the formation of toxic fumes containing hydrogen chloride, volatile heavy metals and ash. This ash invariably contains metal chlorides, organic residues (in particular dioxins and furans), heavy metals and unburnt materials, in which aluminium metal is often found. Before being disharged into the atmosphere, these fumes must consequently be freed of dust and purified.
European patent EP-0,603,218 [Solvay (Societe Anonyme)] describes a process designed to purify such fumes and simultaneously to produce an aqueous sodium chloride solution which can be used in an industrial process. According to this known process, after they have been freed of dust, the fumes are purified to remove hydrogen chloride and volatile heavy metals. To this end, they are treated with sodium bicarbonate in order to decompose the hydrogen chloride and to form sodium chloride, and they are then subjected to a filtration in which a solid residue comprising the sodium chloride and the heavy metals is collected. In order to obtain the industrial aqueous sodium chloride solution, the solid residue is dispersed in water, the aqueous medium thus obtained is basified in order to precipitate the heavy metals in the form of metal hydroxides, the precipitate formed is separated out and the resulting aqueous sodium chloride solution is treated on a chelating resin.
In this known process, the ash separated from the fumes in the dust-removal step (which precedes the removal of hydrogen chloride) generally contains water-soluble chlorine-containing compounds (in particular sodium chloride), heavy metals (in particular zinc, lead, cadmium, mercury and chromium), organic derivatives and unburnt materials (in which aluminium metal can be found). The presence of water-soluble substances, heavy metals and toxic organic materials (dioxins and furans) can pose difficulties for the disposal of this fly ash and involves subjecting it beforehand to an inertization process intended to make it harmless to the environment. An effective inertization process is described in international patent application PCT/EP 97/00778 [Solvay (Societe Anonyme)].